After SCTV, Rosato moved with Duke to the cast of Saturday Night Live for the 1981–1982 season, becoming the first cast member to be born outside of North America. Following Jean Doumanian's tenure during the sixth season and Dick Ebersol trying to retool the show (and relying on Eddie Murphy and Joe Piscopo to spice up the sketches), Rosato only appeared on the show for one year before leaving due to an expired contract.

Rosato next emerged on the Canadian police drama Night Heat, playing Arthur 'Whitey' Morelli. This role lasted from 1985 through 1989. In autumn of 1990, Rosato portrayed Nintendo's character Luigi on DiC's television show The Adventures of Super Mario Bros. 3 (loosely based around the video game of the same title). He continued his role as Luigi in 1991 for the Super Mario World cartoon. He continued to appear in television and film regularly throughout the 1990s.

On May 5, 2005, Rosato was arrested and charged with criminal harassment of his wife Leah, who asserted that his deteriorating mental health had caused her to fear for her safety and that of the couple's infant daughter.[2] The charge was laid after Rosato complained repeatedly to Toronto and Kingston police that his wife, who had recently left him, and daughter had been abducted and replaced by impostors, a belief characteristic of Capgras delusion, a delusional misidentification syndrome with which the Crown's expert psychiatrist had diagnosed Rosato, according to Rosato's eighth lawyer, Daniel Brodsky. It was alleged that the harassment occurred from December 28, 2003 and escalated until April 21, 2005. In spite of the diagnosis, Rosato, who denied mental illness and refused to plead insanity, was held for over two years without bail at a maximum-security detention centre. Daniel Brodsky, who called his client's two-year detention awaiting trial "shocking," asserted that Rosato "spent more time in custody on a harassment charge" than anyone ever convicted of the offence in Canada, estimating that "on average, someone convicted of criminal harassment spends one day in jail and two years on probation." The trial finally commenced on August 7, 2007, in Kingston and it ended on September 5, 2007. In the end, the prosecution downgraded the charge to a summary offence from an indictable offence. Rosato was spared a criminal conviction and handed a conditional discharge, including a psychiatric hospital residence order, issued for a maximum of three years. Rosato was released from the hospital in December 2009 but remained on probation until September 2010.[3]